The present invention relates to a radio receiver having a front end filter comprising helical resonators.
The use of filters having helical resonators in radio frequency applications represents a well known technology. These devices can be used in the 10 MHz to 1.5 GHz region with typical Q's from 200 to 5000. Currently such devices are being used as high Q bandpass filters, band rejection filters, reference cavities, tuning elements for oscillators, front end filters for receivers, and for many other applications where high Q is paramount and large size prohibitive. A conventional helical filter consists of a number of helical resonators enclosed in a rectangular or circular shield, each consisting of a quarter-wavelength transmission line wound in the form of a single layer helix on a circular former, the helical transmission lines being wound in the same direction, short-circuited at the same end (to the shield) and open circuited at the other. The resonators are coupled to each other via openings in partitions between them, the size and location of the openings affecting the coupling coefficient between the resonators and thereby the frequency response of the filter. A more detailed discussion on helical filters can be found in A. I. Zverev's "Handbook of Filter Synthesis". Wiley and Sons 1967.
A disadvantage of using conventional helical resonators in the front end filter of a radio receiver is that in order to obtain satisfactory image frequency rejection many conventional helical resonators are needed, involving significant costs. A further disadvantage of using conventional helical resonators is that the amount of coupling is controlled by the size and location of the openings in the partitions which must be accurately machined, again involving significant costs.